The physical world appears to be a terribly complex place, with many different kinds of particles reacting with each other in a great variety of ways. Yet, scientists are convinced that this apparent " messiness" is an illusion and that fundamentally, the physical world operates on only a few basic principles. The search for underlying simplicity has gone on since the days of ancient Greece.

The first breakthrough in that search occurred in the 1860s when James Clerk Maxwell showed the relationship between two apparently independent forces, electricity and magnetism. Maxwell developed a series of mathematical equations that could be used to describe light and, later, radio waves, x-rays, and other forms of electromagnetic radiation.

During this century, scientists have become convinced that only four fundamental forces exist in nature. In addition to the familiar forces of electromagnetism and gravitation, these include the strong and weak forces...